fixup forbid mandatory named params after optional positional ones

Author: leonerd

op.c

@@ -16990,6 +16990,11 @@ Perl_subsignature_append_named(pTHX_ const char *paramname, PADOFFSET padix, OPC
         return;
     }
 
+    if(!defexpr && signature->opt_params) {
+        yyerror("Mandatory parameter follows optional parameter");
+        return;
+    }
+
     SV *namesv = newSVpvn_utf8(paramname, strlen(paramname), true);
 
     for(size_t parami = 0; parami < signature->named_params; parami++) {

pod/perldiag.pod

@@ -3959,6 +3959,10 @@ caller to omit an earlier one and pass a later one.  If you want to act
 as if the parameters are filled from right to left, declare the rightmost
 optional and then shuffle the parameters around in the subroutine's body.
 
+When using named parameters, no named parameter may be made mandatory if
+there exist any optional I<positional> parameters, though any named
+parameters may be freely mixed between mandatory and optional.
+
 =item Matched non-Unicode code point 0x%X against Unicode property; may
 not be portable
 

pod/perlsub.pod

@@ -506,6 +506,14 @@ The defaulting expressions for optional named parameters are invoked in order
 left to right, as required.  As with positional parameters, each expression
 can see and make use of the values assigned to any previous parameter.
 
+If the subroutine already declares any optional positional parameters, then
+all named parameters must also be optional.  It would not make sense to
+require a mandatory named parameter after any optional positional ones, as
+there would be no way for the caller to provide a value for it.  However, if
+the subroutine does not declare any optional positional parameters, then any
+named ones may be freely mixed between mandatory and optional, since they are
+processed by name and not position.
+
 After positional or named parameters, additional arguments may be captured
 in a slurpy parameter.  The simplest form of this is just an array variable:
 

t/lib/croak/signatures

@@ -87,6 +87,12 @@ sub f (:$x, :$x) { }
 EXPECT
 Duplicated subroutine parameter name at - line 3, near ":$x) "
 ########
+# NAME mandatory named after optional positional
+no warnings 'experimental::signature_named_parameters';
+sub f ($x = 1, :$y) { }
+EXPECT
+Mandatory parameter follows optional parameter at - line 3, near ":$y) "
+########
 sub t061 (@a, @b) { }
 EXPECT
 Multiple slurpy parameters not allowed at - line 2, near "@b) "